Seek recovery using different servo pulse data qualifications modes

ABSTRACT

Apparatus and method for compensating for servo data distortion in a disc drive. During a seek wherein a head is moved from an initial track to a destination track on a disc surface, servo position data are decoded on intermediary tracks by generating nominally redundant positive and negative digital pulse streams from positive and negative peaks in a servo readback signal. The positive and negative digital pulse streams are combined using a default Boolean logical operator (such as an AND operator) to qualify pulses in a first output digital pulse stream used to determine head position. When distortion arises that prevents correct detection of head position from the first output digital pulse stream, a second Boolean logical operator (such as an OR operator) is switched in to generate a second output digital pulse stream. Depending on the type of distortion, a selected one of the operators will provide improved servo data decoding.

RELATED APPLICATIONS

This application claims priority to U.S. Provisional Application No.60/121,101 filed Feb. 22, 1999.

FIELD OF THE INVENTION

This invention relates generally to the field of disc drive data storagedevices, and more particularly, but not by way of limitation, toimproving servo performance of a disc drive through the use of differentpulse stream combinations to compensate for head construction andperformance variations.

BACKGROUND

Disc drives are commonly used as the primary data storage and retrievaldevices in modern computer systems. In a typical disc drive, user dataare magnetically stored on one or more discs that are rotated at aconstant high speed and accessed by a rotary actuator assembly having aplurality of read/write heads that fly adjacent the surfaces of thediscs. A read channel and interface circuit are provided to recoverpreviously stored data from the discs to a host computer. A closed loopdigital servo system provide head positional control using servo datawritten to the discs during manufacturing.

The servo system carries out two primary operations: seeking and trackfollowing. A seek involves moving a selected head from track to track,while track following involves causing the head to follow a particulartrack. It will be noted that data reading and writing operationsgenerally occur while the disc drive is in the track following mode.

A typical seek operation uses a velocity controlled approach wherein avelocity profile, indicative of a desired velocity trajectory for thehead, is selected based on the distance between an initial track and adesired, destination track. For a seek of sufficient length, the head isfirst accelerated to a maximum velocity, maintained at this maximumvelocity until it reaches a predetermined distance from the destinationtrack, and then decelerated to settle onto the destination track.Adjustments in the velocity of the head are repetitively made during theseek in relation to the difference between the actual velocity and thedesired velocity trajectory of the velocity profile.

Since the various desired velocity values that make up the velocityprofile are applied in relation to the number of tracks to go to thedestination track, it is necessary for the servo system to keep track ofthe position of the head as the head crosses each of the intermediatetracks during the seek. This is typically carried out by tranducing theservo data to identify each track crossing as the head is moved towardthe destination track. Should the servo system become unable tocorrectly detect the servo data, the servo system will abort the seeksince closed-loop head positional control will be lost at that point.

As will be recognized, improved data transfer performance levels havebeen achieved in recent generations of disc drives through theintroduction of magneto-resistive (MR) heads. An MR head incorporatesseparate write and read elements, with the write element comprising aninductive coil about a core with a write gap and the read elementcomprising a magneto-resistive material having a changed electricalresistance in the presence of a magnetic field of selected orientation.

The increased sensitivity of the MR element allows write pulses from thehead to be relatively narrow, facilitating higher data transfer rates.However, due to the complexity and minute dimensions of MR heads, it isbecoming increasingly difficult to design and manufacture heads thatexhibit flawless operation. While manufacturing screening operationsattempt to sort flawed heads and remove such from the manufacturingprocess, some heads nevertheless exhibit intermittent discrepantoperation, which is usually manifested as temporary distortion in thereadback signal.

Ordinarily, a distorted data readback signal may be corrected withappropriate error correction code (ECC) based techniques. However, suchECC techniques cannot be applied to the position feedback-based servodata. Thus, while MR heads have provided significant gains in datatransfer performance, such heads can cause intermittent degradations inservo performance which are not easily correctable. It has been foundthat some MR heads exhibit a greater tendency to output distortedreadback signals upon initialization, before steady-state thermalequilibrium conditions within the heads have been are achieved. Thus,distortion can often occur when the disc drive switches between a first“warm” head to a second “cool” head and immediately attempts to executea seek operation with the second head.

Accordingly, there is a need for improvements in the art to enable discdrives to correctly decode servo data in the presence of signaldistortion from an MR head. It is to such improvements that the presentinvention is directed.

SUMMARY OF THE INVENTION

The present invention provides an apparatus and method for improvingdisc drive servo control performance.

In accordance with preferred embodiments, a disc drive includes a headadjacent a rotatable disc having a surface on which a plurality oftracks are defined by servo position data. A seek is carried out to movethe head from an initial track to a destination track using a velocitycontrolled approach wherein currents are applied to an actuator motor inrelation to distance to go to the destination track.

As the head is moved toward the destination track, a servo circuitdecodes servo position data stored on intermediary tracks between theinitial track and the destination track by transducing an analog servoreadback signal from the servo position data, the servo readback signalhaving successive pairs of positive and negative peaks. A positivedigital pulse stream is generated with pulses indicative of positiveamplitude peaks in the servo readback signal. Similarly, a negativedigital pulse stream is generated with pulses indicative of negativeamplitude peaks in the servo readback signal. The positive and negativedigital pulse streams are thereafter combined using a first Booleanlogical operator to form a first output digital pulse stream nominallyindicative of position of the head.

When the servo circuit determines that first output digital pulse streamfails to correctly indicate the position of the head, typically as aresult of distortion in the servo readback signal, the servo circuitselects a second Boolean logical operator to form a second outputdigital pulse stream from the positive and negative digital pulsestreams. While different combinations of Boolean operators can be usedto obtain similar results, preferably, a selected one of the Booleanlogical operators is an AND operator and the remaining Boolean logicaloperator is an OR operator.

An AND operator typically provides improved results in the presence ofspurious peaks (drop ins) in the servo readback signal caused by, forexample, baseline shifts. An OR operator typically provides improvedresults when peaks are missing (drop outs) from the digital pulsestreams, caused by, for example, asymmetry in the servo readback signal.

Preferably, separate logical mode values are stored in a table by headand updated as required during successive seeks for each head. In thisway, the servo data are decoded in relation to the logical operator thathas most recently been found to provide better servo performance.

These and various other features and advantages which characterize thepresent invention will be apparent from a reading of the followingdetailed description and a review of the associated drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a top plan view of a disc drive constructed in accordance withpreferred embodiments of the present invention.

FIG. 2 is a functional block diagram of a servo circuit of the discdrive of FIG. 1.

FIG. 3 illustrates the manner in which servo data are stored on each ofthe disc recording surfaces of the disc drive.

FIG. 4 is a representation of a selected servo block from FIG. 3.

FIG. 5 provides a graphical illustration of the manner in which servodata are decoded by the servo circuit of FIG. 2.

FIG. 6 is a functional block diagram generally illustrating the mannerin which the positive and negative pulse streams of FIG. 5 arealternatively combined to generate the output pulse stream of FIG. 5.

FIG. 7 provides a graphical illustration of the manner in which servodata are decoded by the servo circuit of FIG. 2 in the presence of servoreadback signal asymmetry (dropouts).

FIG. 8 provides a graphical illustration of the manner in which servodata are decoded by the servo circuit of FIG. 2 in the presence of servoreadback signal baseline shift (drop ins).

FIG. 9 is a flow chart for a SEEK RECOVERY routine, illustrative ofsteps carried out in accordance with preferred embodiments of thepresent invention.

DETAILED DESCRIPTION

Referring to FIG. 1, shown therein is a top plan view of a disc drive100 of the type used to interface with a host computer to magneticallystore and retrieve user data. The disc drive 100 includes a base deck102 and a top cover 104 (shown in partial cutaway fashion) whichcooperate to form a housing in which various components are mounted.

A spindle motor 106 rotates a plurality of axially-aligned, rigid,magnetic recording discs 108 at a constant high speed in an angulardirection denoted by arrow 109. User data are written to and read fromtracks (not designated) on the discs 108 through the use of an actuatorassembly 110, which rotates about a bearing shaft assembly 112 adjacentthe discs 108. The actuator assembly 110 includes a plurality of rigidactuator arms 114 which support flexible suspension assemblies 116(flexures). Mounted at the distal end of each of the flexures 116 is ahead 118 used to magnetically access the corresponding disc surface. Theheads 118 are preferably characterized as magneto-resistive (MR) headseach having a thin-film inductive write element and an MR read element.

When the disc drive 100 is not in use, the heads 118 are parked onlanding zones 120 and the actuator assembly 110 is secured using amagnetic latch assembly 122. The radial position of the heads 118 iscontrolled with a voice coil motor (VCM) 124 having a coil 126 whichinteracts with a magnetic circuit that includes a permanent magnet 128.A flex assembly 130 facilitates electrical communication between theactuator assembly 110 and a disc drive printed circuit board (PCB)attached to the underside of the base deck 102. The flex assembly 130includes a preamplifier/driver circuit 132 (preamp) which applies writecurrents to the write elements of the heads 118 during a write operationand applies read bias currents to the MR read elements of the heads 118during a read operation.

FIG. 2 shows a functional block diagram of a servo circuit 140 of thedisc drive 100 which uses servo data arranged on the discs 108 tocontrol the position of the heads 118. The servo data are transduced bythe selected head 118, preamplified by the preamp 132, and provided to ademodulation circuit 142 which conditions the servo data for processingby a servo processor 144, which preferably comprises a digital signalprocessor (DSP). The DSP 144 uses programming steps stored in DSP memory146 (MEM) as well as commands issued by a top-level disc drive processor(not shown) to output current command signals to coil driver circuitry148 which, in turn, adjusts the amount of current applied to the coil126.

The servo data are preferably arranged on the discs 108 as shown inFIGS. 3-4. FIG. 3 provides a portion of a selected one of the discs 108with a plurality of servo wedges 150 which radially extend from theinnermost to the outermost radii of the disc recording surface. Theservo wedges 150 are written during disc drive manufacturing and eachservo wedge comprises a plurality of radially aligned servo fields, witheach servo field corresponding to a particular track on the discsurface. As will be recognized, user data are stored in the areasbetween adjacent servo wedges.

FIG. 4 shows a portion of a track 152 at a selected radius on the disc108, illustrating the arrangement of respective servo fields 154 anduser data fields 156. Each servo field 154 preferably includes anautomatic gain control (AGC) field 158, an index field 160, a Gray code(GC) field 162 and a position (POS) field 164. The AGC field 158provides an oscillating signal that prepares the servo circuit 140 forremaining portions of the servo field 154, the GC field 162 provides aunique track address to indicate radial position for the track, and thePOS field 164 provides an arrangement of servo burst patterns thatallows the servo circuit 140 to perform intra-track positioning. It willbe apparent that other servo field configurations can be readilyemployed.

FIG. 5 illustrates the manner in which the servo data from the servofields 154 are decoded by the demodulator 142 of FIG. 2. A number ofsignal waveforms are plotted in FIG. 5 against a common x-axis 166indicative of elapsed time and a common y-axis 168 indicative ofrespective signal amplitudes.

The first signal waveform in FIG. 5 is a servo readback signal 170representative of a readback signal generated by the read element of theselected head 118 as a portion of one of the servo fields 154 passesunder the head during a seek. It will be noted that the servo circuit140 of FIG. 2 can be configured as desired to rely on all of the servodata, or only portions of the data, from the servo fields 154 to detecteach track crossing as the head 118 is moved from the initial track tothe destination track during the seek. Thus, for purposes ofillustration it will be contemplated that the servo readback signal 170of FIG. 5 represents the transducing of the Gray code field 162 of FIG.4, although the present invention is not so limited.

The servo data are written as a series of di-bit pulse pairs of opposingmagnetization so that, when transduced, the resulting readback signalprovides a peak amplitude of one polarity immediately followed by asecond peak amplitude of the opposite polarity. For a more detaileddiscussion of recording servo data using di-bit pulse pairs, see U.S.Pat. No. 5,801,896 issued Sep. 1, 1998 to Frietas. In the present case,the servo data are written so that each di-bit pulse pair produces apositive peak followed by a negative peak, such as denoted at peaks 172,174 respectively, but it will be understood that the servo data couldreadily be written in opposite fashion.

Positive and negative peak detection thresholds 176, 178 are selectedwith appropriate values such as set forth by U.S. Pat. No. 6,055,121issued Apr. 25, 2000 to Du. The thresholds 176, 178 are applied to theservo readback signal 170 to generate a positive pulse stream 180 and anegative pulse stream 182, respectively. The positive and negative pulsestreams 180, 182 generally comprise digital signals with a first value(such as logical one) when the servo readback signal 170 exceeds therespective threshold 176, 178 and a second value (such as logical zero)at all other times. Because a negative pulse follows each positive pulsein the readback signal 170, the negative pulse stream 182 will nominallyexhibit the same informational content as the positive pulse stream 180,but the negative pulse stream 182 will be delayed by one-half cycle(i.e., the time between successively occurring pulses).

The positive pulse stream 180 is thus preferably delayed by acorresponding amount to produce a delayed positive pulse stream 184, asshown, with the delayed positive pulse stream 184 nominally in phasewith the negative pulse stream 182. The use of dual, redundant pulsestreams enhances the ability of the servo system to properly decode theservo data, as discussed below. At this point, however, it will be notedthat the two pulse streams 182, 184 are thereafter combined using aselected logical operator (such as an AND or OR) to produce an outputpulse stream 186 having the peak detection content of the servo readbacksignal 170 and which, after further conditioning, is used by the DSP 144as a positional input.

FIG. 6 provides a logical block diagram for a portion of the demodulator142 of FIG. 2, illustrating the manner in which the output pulse stream186 is preferably generated. It will be understood that the functionaloperations of FIG. 6 can be carried out in a number of ways, includingthrough the use of hardware or firmware, as desired.

The delayed positive pulse stream 184 and the negative pulse stream 182are routed via respective signal paths 188 and 190 to a pair of switches192, 194. Outputs from the switches 192, 194 are routed to an AND gate196 and an OR gate 198, respectively, the outputs of which are tiedtogether to a common output path 200 on which the output pulse stream186 is transmitted.

The switches 192, 194 are characterized as “normally open” type switcheswhich close the respective connections between the input and outputsignal lines when a logical one is present at the respective selection(SEL) inputs. A pulse data qualification table 202 thus selects thelogical operator (whether AND or OR) used to combine the signals 182,184 by outputting a logical mode value of one to select an AND and alogical zero to select an OR (the logical zero is inverted to become alogical one by operation of inverter 204). As will be recognized, theAND logical operator will provide a pulse in the output pulse stream 186when a corresponding pulse appears in both the negative and delayedpositive pulse streams 182, 184, and the OR logical operator willprovide a pulse in the output stream 186 when a corresponding pulseappears in either of the two streams 182, 184. The pulse dataqualification table 202 preferably stores a different logical mode valuefor each head, with the contents of the pulse data qualification table202 being supplied to the demodulator 142 by the DSP 144. The manner inwhich the particular logical mode values are set for each head will bediscussed in detail below.

At this point, however, it will be helpful to briefly discuss examplesof different types of servo signal distortion that can be experienced bythe disc drive 100 under various conditions. FIG. 7 illustrates theoperation of the demodulator 142 in the presence of servo readbacksignal distortion characterized as signal peak asymmetry. Moreparticularly, servo readback signal 210 is nominally identical to theservo readback signal 170 of FIG. 5 except that the signal 210 in FIG. 7includes an asymmetric negative peak pulse 212, which has an amplitudethat is less than than the corresponding negative peak detectionthreshold 178.

Hence, while a positive pulse stream 214 includes all six of thepositive peaks in the servo readback signal 210, a negative pulse stream216 only includes five of the negative peaks in the servo readbacksignal 210 (the missing sixth negative pulse is shown in broken linefashion at 218). The missing pulse 218 is referred to as a “drop out” inthe negative pulse stream signal 216. It will be noted, that asymmetricpeaks such as as 212 can be either positive or negative in amplitude.

In the example of FIG. 7, selecting a logical OR as the logical operatorin FIG. 6 to combine the negative pulse stream 216 and a delayedpositive pulse stream 220 will provide an output pulse stream 222 withthe desired informational content.

FIG. 8 provides another illustration of the operation of the demodulator142, this time in the presence of servo readback signal distortion inthe form of baseline shift. Particularly, servo readback signal 230includes a shifted baseline portion 232 which exceeds the positivethreshold 176, thereby generating an additional, undesired pulse 234 ina positive pulse stream 236. This additional pulse 234 is referred to asa “drop in” in the positive pulse stream 236. A corresponding negativepulse stream 238 has the correct informational content since thebaseline shift of FIG. 8 is shown to be in the positive direction. Suchbaseline shift, however, can can be either positive or negative.

In this case, a logical AND is the better choice to combine the negativepulse stream 238 and a delayed positive pulse stream 240, since aresulting output pulse stream 242 will not include a pulse correspondingto the baseline shifted portion 232.

Referring again to the servo circuit 140 of FIG. 2, during a seekoperation for a selected one of the heads 118, the servo circuit 140determines the address of the track being followed by the selected head(from the GC field), determines the address of the destination track,and calculates the distance therebetween in terms of tracks to go.Thereafter, the servo circuit 140 initiates a seek to move the selectedhead 118 from the initial track to the destination track by applyingcurrent to the coil 126 to accelerate the head 118 in the appropriatedirection. During the seek, the velocity of the head is controlled withrespect to position, with the servo data on intermediary tracks beingdetected to continually determine the position of the head 118. At aselected distance from the destination track, current is applied to thecoil 126 of opposite polarity to decelerate the head 118 to come to restover the destination track in accordance with a deceleration profilewhich defines the desired deceleration velocity trajectory for the head118. It will be understood that during the detection of the servo datafrom intermediary tracks, the negative and delayed positive pulsestreams (such as 182, 184 in FIG. 5) are combined in accordance with thepreselected logical operator (i.e., either AND or OR) for the selectedhead, as provided by the pulse data qualification table 202.

Should the readback signal from the selected head 118 become distorted,however (such as illustrated in FIGS. 7 and 8), the servo circuit 140will generally be unable to determine with certainty the position of thehead during the seek. In the prior art, typically the only way toresolve such a situation has been to abort and retry the seek asuccessive number of times until the problem is resolved, or a seekerror is declared to the host.

The present invention, however, as embodied herein and claimed below,provides an improved approach to recovering from intermittent positiondata readback signal distortion during a seek. FIG. 9 provides a SEEKRECOVERY routine 250, illustrative of steps carried out in accordancewith preferred embodiments of the present invention. It will beunderstood that the flow of FIG. 9 preferably represents programmingstored in DSP memory 146 and utilized by the DSP 144 during a seek.

At step 252, the servo circuit 140 initiates a seek with a selected head118 to a destination track, generally in accordance with the foregoingdiscussion. Thus, step 252 includes the periodic generation of servoreadback signals (such as the signal 170 of FIG. 5) from the servofields 154 on the intermediary tracks between the initial anddestination tracks. Step 252 further includes the generation of positiveand negative pulse streams (such as 182, 184 of FIG. 5) from thereadback signals and the use of the then-existing logical mode value(“first” value) from the table 202 to combine the streams to generatethe output pulse stream (such as 186) for use by the DSP 144.

As long as the servo readback signals are well behaved (nondistorted)such as the signal 170 in FIG. 5, the particular operator (whether ANDor OR) used as the first logical mode value will not have much effect,if any, upon the characteristics of the output pulse stream 186. Thatis, for a nondistorted readback signal, both an AND and an OR operatorwill generally result in the same output pulse stream 186 (exceptingminor edge timing variations), and the output pulse stream 186 will notbe distorted, i.e., it will not include missing pulses (dropouts) oradditional, undesired pulses (drop ins).

However, should distortions arise in the servo readback signal, thefirst logical mode value from the table 202 may or may not be able toadequately qualify the pulses in the output pulse stream during theseek. For example, with reference again to FIG. 7, an asymmetric pulsesuch as 212 in the servo readback signal will still result in a correctinformational content in the output pulse stream 186 if an OR logicalmode value is used, but an AND logical mode value would result in amissing pulse; that is, pulse 254 would not appear in the output pulsestream 222. Likewise, with reference to FIG. 8, the baseline shiftedportion 232 in the servo readback signal 230 results in the extra pulse234 in the positive pulse stream 236. This extra pulse 236 does notappear in the output pulse stream 242 using an AND logical mode value.Using an OR logical mode value, however, would undesirably provide anadditional pulse 256 (shown in broken line fashion) in the output pulsestream 242.

Accordingly, decision step 258 determines whether distortion is presentin the output pulse stream (such as in the form of a missing pulse or anextra, unexpected pulse) sufficient to prevent the servo circuit 140from properly determining the position of the head 118 during the seek.See, for example, U.S. Pat. No. 5,801,896, which teaches the decoding ofa servo seek mode tracking signal. If no such distortion occurs duringthe seek, the servo circuit 140 will be able to properly determine theposition of the head during the entirety of the seek until the head 118reaches the destination track. In such case the flow passes to step 260wherein the head 118 is settled onto the destination track, after whichthe disc drive 100 proceeds to carry out the desired data transferoperation thereon (and the routine ends at 262).

The foregoing sequence represents the typical operation of the discdrive 100, as servo data distortion is contemplated as generallyoccurring only on an intermittent and infrequent basis. If thedistortion is related to temperature, it is contemplated that thedistortion may have a greater probability of arising when the head 118is initially selected; that is, prior to the seek operation of step 252,a different head 118 adjacent a different data recording surface wasbiased by the preamplifier and a head switch operation was carried outto switch from this prior head to the selected head.

On the other hand, when servo data distortion does arise during the seeksufficient to prevent the servo circuit 140 from decoding the servodata, the routine passes from decision step 258 to step 264 at whichpoint the head is immediately brought to rest. This is preferablycarried out by applying a temporary electrical short across the actuatorcoil 126. The purpose of bringing the head 118 to rest is precautionaryin nature; once the position of the head 118 becomes unknown, continuingto allow the actuator 110 to move could potentially result in theinadvertent slamming of the actuator against an inner or outer limitstop at a terminal velocity sufficient to damage the heads 118.

Once the head 118 is brought to rest, the servo circuit 140 switches toa different, “second” logical mode value for the head 118 at step 266.If AND was previously used, step 266 changes this to OR, and vice-versa.The DSP 144 preferably writes the updated logical mode value to thetable 202 so that the updated value is provided to the demodulator 142for subsequent use.

After the second logical mode value has been selected, the flowcontinues to step 268 where the position of the head 118 is acquired bytransducing the servo data from the associated track to which the head118 has come to rest. The routine then passes to step 270 wherein a seekis again initiated to the destination track, this time using the secondlogical mode value selected in step 266. It is contemplated that, inmost cases, the toggling of the logical mode value will enable the servocircuit 140 to correctly decode the servo data and complete the seek.However, for purposes of completeness decision step 272 has beenincluded to show an inquiry whether distorted servo data again arisesduring the seek using the second logical mode value; if so, the routinepasses to step 274 wherein other corrective actions are applied by thedisc drive 100 (such as adjustments in gain and threshold values, etc.).

Preferably, the contents of the pulse data qualification table 202 areinitially set and thereafter adjusted on a head-by-head basis duringrespective seeks arrive at optimum values based on which modes providebetter servo performance. Although the logical mode values of the table202 have been described in terms of seeking, it will be understood thatall servo data demodulation operations for a particular head are carriedout using the particular mode identified in the table 202. Finally,depending upon the requirements of a given application, the logical modevalues can further be assigned on a per-head, per-zone basis, so thatdifferent logical mode values are utilized by the same head at differentradii on the disc.

In summary, it will now be recognized that the present invention isdirected to an apparatus and method for compensating for servo datadistortion during a disc drive seek. In accordance with preferredembodiments, a disc drive 100 includes a head 118 adjacent a rotatabledisc 108 having a surface on which a plurality of tracks are defined byservo position data 154. A seek is carried out to move the head from aninitial track to a destination track using a velocity controlledapproach wherein currents are applied to an actuator motor 124 inrelation to distance to go to the destination track.

As the head is moved toward the destination track, a servo circuit 140decodes servo position data stored on intermediary tracks between theinitial track and the destination track by transducing an analog servoreadback signal 170, 210, 230 from the servo position data, the servoreadback signal having successive pairs of positive and negative peaks.A positive digital pulse stream 180, 214, 236 is generated with pulsesindicative of positive amplitude peaks in the servo readback signal.Similarly, a negative digital pulse stream 182, 216, 238 is generatedwith pulses indicative of negative amplitude peaks in the servo readbacksignal. The positive and negative digital pulse streams are thereaftercombined using a first Boolean logical operator to form a first outputdigital pulse stream nominally indicative of position of the head.

When the servo circuit determines that first output digital pulse streamfails to correctly indicate the position of the head, which can occur asa result of distortion in the servo readback signal, the servo circuitselects a second Boolean logical operator to form a second outputdigital pulse stream from the positive and negative digital pulsestreams. Preferably, a selected one of the Boolean logical operators isan AND operator and the remaining Boolean logical operator is an ORoperator. An AND operator typically provides improved results in thepresence of spurious peaks (drop ins) in the servo readback signalcaused by, for example, baseline shifts. An OR operator typicallyprovides improved results when peaks are missing (drop outs) from thedigital pulse streams, caused by, for example, asymmetry in the servoreadback signal.

As used herein, the function of “moving the head from an initial trackto a destination track on the disc surface” will be understood as beingcarried out by the disclosed servo circuit 140 including demodulator 142configured as shown in FIG. 6 and processor 144 programmed to operate inaccordance with the flow of FIG. 9. Structures that do not (a) combinepositive and negative pulse streams in accordance with a first Booleanlogical operator to form a first output pulse stream, (b) determine thatthe first output pulse stream fails to correctly identify head position,and then (c) as a result switch to a second, different Boolean logicaloperator to combine the positive and negative pulse streams to form asecond output pulse stream are expressly excluded from the definition ofan equivalent structure.

It will be clear that the present invention is well adapted to attainthe ends and advantages mentioned as well as those inherent therein.While a presently preferred embodiment has been described for purposesof this disclosure, numerous changes may be made which will readilysuggest themselves to those skilled in the art and which are encompassedin the spirit of the invention and as defined in the appended claims.

What is claimed is:
 1. In a disc drive having an actuator whichpositions a head adjacent a plurality of tracks on a rotatable discsurface, a method for carrying out a seek operation wherein the head ismoved from an initial track to a destination track, comprising steps of:(a) accelerating the head in a direction toward the destination track;(b) decoding servo position data stored on intermediary tracks betweenthe initial track and the destination track by transducing an analogservo readback signal from the servo position data, generating apositive digital pulse stream with pulses indicative of positiveamplitude peaks in the servo readback signal, generating a negativedigital pulse stream with pulses indicative of negative amplitude peaksin the servo readback signal, and combining the positive and negativedigital pulse streams to form a first output digital pulse stream inaccordance with a first Boolean logical operator, the first outputdigital pulse stream nominally indicative of position of the head; (c)determining that the first output digital pulse stream fails tocorrectly indicate the position of the head; and (d) combining thepositive and digital pulse streams to form a second output digital pulsestream in accordance with a second Boolean logical operator, the secondoutput digital pulse stream nominally indicative of the position of thehead.
 2. The method of claim 1, wherein a selected one of the first andsecond Boolean logical operators comprises a logical AND operator andthe remaining one of the first and second Boolean logical operatorscomprises a logical OR operator.
 3. The method of claim 1, furthercomprising a step of (e) decelerating the head to bring the head to restat a position between the initial and destination tracks before theoperation of step (d).
 4. The method of claim 3, wherein step (e)comprises applying an electrical short across an actuator coil used tomove the head across the disc surface.
 5. The method of claim 1, whereinstep (b) further comprises steps of applying respective positive andnegative peak detection thresholds to the servo readback signal so thatthe pulses in the positive digital pulse stream have widthscorresponding to portions of the servo readback signal above thepositive peak detection threshold, and so that the pulses in thenegative digital pulse stream have widths corresponding to portions ofthe servo readback signal below the negative peak detection threshold.6. The method of claim 1, further comprising a step of (e) storing alogical mode value indicative of the first logical operator in a memorylocation, wherein the memory location is subsequently updated with a newlogical mode value indicative of the second logical operator.
 7. Themethod of claim 1, wherein step (b) further comprises a step of delayingat least one of the positive and negative digital pulse streams to bringthe respective pulses of the positive and negative digital pulse streamsinto nominal phase alignment.
 8. In a disc drive having a rotatable datarecording surface with a plurality of tracks which are accessed by aread/write head supported by a moveable actuator, a method for carryingout a seek wherein the head is moved from an initial track to adestination track, comprising steps of: (a) generating an analog servoreadback signal having successively occurring pairs of positive andnegative peaks from servo position data transduced as the head is movedacross the data recording surface; (b) applying respective positive andnegative peak detection thresholds to the servo readback signal togenerate respective positive and negative digital pulse streams; (c)generating a first output digital pulse stream nominally indicative ofhead position as a logical combination of the positive and negativedigital pulse streams using a first Boolean logical operator; (d)detecting presence of distortion in the servo readback signal whichprevents the first output digital pulse stream from correctly indicatingthe position of the head; (e) selecting a second Boolean logicaloperator; (f) generating a second analog servo readback signal havingsuccessively occurring pairs of positive and negative peaks from servoposition data transduced as the head is moved between the initial trackand the destination track; (g) applying respective positive and negativepeak detection thresholds to the second servo readback signal togenerate respective second positive and negative digital pulse streams;(h) combining the respective second positive and negative digital pulsestreams using the second Boolean logical operator to generate a secondoutput digital pulse stream nominally indicative of head position. 9.The method of claim 8, further comprising a step of: (I) using thesecond output digital pulse stream to position the head onto thedestination track.
 10. The method of claim 8, wherein a selected one ofthe first and second Boolean logical operators comprises a logical ANDoperator and the remaining one of the first and second Boolean logicaloperators comprises a logical OR operator.
 11. The method of claim 8,further comprising a step of (I) decelerating the head to bring the headto rest at a position between the initial and destination tracks afterthe operation of step (d).
 12. The method of claim 8, further comprisinga prior step of storing a logical mode value indicative of the firstlogical operator in a memory location, wherein the memory location issubsequently updated with a new logical mode value indicative of thesecond logical operator.
 13. The method of claim 8, wherein step (b)further comprises a step of delaying a selected one of the positive andnegative digital pulse streams to bring respective pulses of thepositive and negative digital pulse streams into nominal phasealignment.
 14. A disc drive, comprising: a rotatable data recordingsurface on which a plurality of tracks are defined by servo positiondata; an actuator which supports a head adjacent the data recordingsurface; an actuator motor configured to move the actuator; and a servocircuit which carries out a seek to move the head from an initial trackto a destination track on the data recording surface by applying currentto the actuator motor while detecting head position by transducing ananalog servo readback signal from the servo position data, generating apositive digital pulse stream with pulses indicative of positiveamplitude peaks in the servo readback signal, generating a negativedigital pulse stream with pulses indicative of negative amplitude peaksin the servo readback signal, and combining the positive and negativedigital pulse streams to form a first output digital pulse stream inaccordance with a first Boolean logical operator, wherein the servocircuit switches to a second Boolean logical operator to form a secondoutput digital data pulse stream when the first output digital pulsestream fails to correctly indicate the position of the head.
 15. Thedisc drive of claim 14, wherein a selected one of the first and secondBoolean logical operators comprises a logical AND operator and theremaining one of the first and second Boolean logical operatorscomprises a logical OR operator.
 16. The disc drive of claim 14, whereinthe servo circuit further applies current to the actuator motor to bringthe head to rest at a position between the initial and destinationtracks after determining that the first output digital pulse streamfails to correctly indicate the position of the head.
 17. The disc driveof claim 14, wherein the servo circuit stores a logical mode valueindicative of the first logical operator in a memory location, andwherein the memory location is subsequently updated with a new logicalmode value indicative of the second logical operator.
 18. The disc driveof claim 14, wherein the servo circuit further delays a selected one ofthe positive and negative digital pulse streams to bring respectivepulses of the positive and negative digital pulse streams into nominalphase alignment.
 19. A disc drive, comprising: a head adjacent to arotatable disc surface on which a plurality of tracks are defined; andmovement means for moving the head from an initial track to adestination track on the disc surface.